1. Field of the Invention
The present invention relates to a surface lighting device and a liquid crystal display device including the surface lighting device, and in particular to the surface lighting device which realizes a surface light source by arranging LEDs (Light Emitting Diode), which are capable of emitting light of primary colors of red, green and blue, or LEDs, which emit light of two or more kinds of colors in a complementary color relation, and the liquid crystal display device which attains high luminance even on a large display area by including the surface lighting device.
2. Description of the Related Art
Examples of a backlight unit incorporated in a liquid crystal display device include a sidelight or direct type backlight unit using a cold cathode tube and a sidelight type backlight unit using an LED. However, the cold cathode tube has problems in that high voltage is required for an inverter and that mercury is contained and, to make matters worse, has a problem in that a color reproduction range is narrow.
Thus, it is the mainstream in recent years to use an LED as a light source. Conventional examples will be described below.
A display device 160 of a conventional example 1 shown in FIG. 23 is characterized by including: a diffusion plate 1; a light guide plate 18 which is set behind the diffusion plate 1; and a red light-emitting LED 7, a blue light-emitting LED 8 and a green light-emitting LED 9 which are arranged on the side of the light guide plate 18 and carry out lighting, and in that light from the light-emitting LEDs in the light guide plate 18 is mixed and the light is diffused on the diffusion plate 1, whereby surface lighting is realized (JP-A-2002-341797).
In addition, a liquid crystal display device 170 of a conventional example 2 shown in FIG. 24 is characterized by including: a liquid crystal display panel 13; a prism sheet 11 which is set behind the liquid crystal display panel 13; a first diffusion plate 14 which is set behind the prism sheet 11; a substrate 4 which is set behind the first diffusion plate 14 and is mounted with plural white LEDs 12; and a second diffusion plate 15 which is set behind the substrate 4 and is used for taking in extraneous light, and in that white light from the white LEDs is supplied and extraneous light is taken in to light the liquid crystal display panel two-dimensionally (JP-A-2002-311412).
Moreover, a liquid crystal display device 180 of a conventional example 3 shown in FIG. 25 is characterized by including: a liquid crystal display panel 13; a diffusion plate 1 which is arranged behind the liquid crystal display panel; a light guide plate 18 which is arranged behind the diffusion plate 1; a transparent plate 16 which is set behind the light guide plate 18; a substrate 4 which is set behind the transparent plate 16 and includes red light-emitting LEDs 7, blue light-emitting VFDs (vacuum fluorescent displays) 17, green light-emitting LEDs 9 and a reflection plate 2 which is arranged so as to be laid among the LEDs and the VFDs; and sidelights 20 which are arranged on the sides of the light guide plate 18, and in that the red light-emitting LEDs 7, the blue light-emitting VFDs 17 and the green light-emitting LEDs 9 can emit light independently for each single color and are lit almost simultaneously in a short time and in order for obtaining white light (JP-A-6-018882, JP-A-2002-258815). In addition, in the liquid crystal display device 180 of the conventional example 3, light from the LEDs is transmitted through the transparent plate 16 to be made incident on the light guide plate 18, mixed, and diffused on the diffusion plate 1, whereby surface lighting is supplied to the liquid crystal display panel 13. Further, the reflection plate 2 is useful for returning the light, which has returned from the diffusion plate 1 or the transparent plate 16, to the liquid crystal display panel 13 again, and increasing luminance of the surface lighting. Here, the reflection plate 2 is set so as to avoid a metal-embedded PCB 5, on which a circuit for driving light-emitting elements such as the red LED 7 or wiring for sending signals is disposed, as structural example 2 of an LED part and a reflection plate in the conventional example 3 shown in FIG. 26.
In the display device 160 of the conventional example 1, since LEDs are arranged at corners or sides of a light guide plate, there is a problem in that high luminance cannot be attained on a large display area.
In addition, in the liquid crystal display device 170 of the conventional example 2, white LEDs are used, but the white LEDs have a problem in that the range of color reproducibility is narrow, since the white is made by emitting blue color light from blue LCD to a yellow fluorescent material.
Moreover, in the surface lighting of the liquid crystal display 180 of the conventional example 3, since LEDs of different colors are used, there is a problem in that unevenness of colors and unevenness of luminance due to unbalance of amounts of light among the LEDs, which emit light of different colors, cannot be overcome easily.